1. Field of the Invention
The present invention relates to an image forming apparatus, in particular, relates to a configuration to correct a side end position of a sheet.
2. Description of Related Art
In the related art, there has been an image forming apparatus such as a copying machine, a printer and a facsimile machine provided with a sheet processing portion to perform a process such as stapling, aligning and binding against image-formed sheets while a sheet cassette as a sheet accommodation portion is detachably attached to an image forming apparatus main body. When forming an image, a sheet accommodated in the sheet cassette is fed by a sheet feeding roller and conveyed to an image forming portion. Then, the image-formed sheet is conveyed to a fixing portion and the image is fixed on the sheet. Subsequently, the image-fixed sheet is conveyed to the sheet processing portion and the process such as binding is performed.
Here, there has been an image forming apparatus provided with a skew-feeding correction portion to perform correction of sheet skew feeding and sheet displacement in the direction perpendicular to the sheet conveying direction (hereinafter, called the width direction) in order to adjust the posture and position of a sheet. As such a skew-feeding correction portion, a configuration to perform correction of a displacement of sheet in the width direction by a pair of slidable registration rollers has been disclosed in U.S. Pat. No. 4,799,084.
FIG. 10 is a view which illustrates the configuration of a registration device arranged at an image forming apparatus capable of performing multiple transferring having a pair of slidable registration rollers. When performing multiple transferring onto a sheet, the registration device performs correction of the displacement in the width direction by utilizing the pair of slidable registration rollers 40 so that color shifting between the first color and the second color is accurately corrected.
In the registration device, when a sheet is conveyed, skew feeding of the sheet top end is corrected by firstly hitting the sheet top end to the pair of registration rollers 40 slidable in the axis direction and forming a loop with a conveying roller 36 at the upstream side. Next, the sheet of which skew feeding is corrected is moved to slide in the axis direction while being conveyed by the pair of registration rollers 40, and then, the side end of the slide-moved sheet is detected by an optical sensor 61. Then, the image of the first color is formed on the sheet moved to the position where the side end is detected by the optical sensor 61.
Next, when forming the image of the second color, the sheet is conveyed to the pair of registration rollers 40 again and skew feeding is corrected after the image of the first color is formed. Subsequently, the sheet of which skew feeding is corrected is moved to slide while being conveyed by the pair of registration rollers 40, and then, the side end of the slide-moved sheet is detected by the optical sensor 61. Accordingly, even in the case of forming the second color image, the displacement of sheet in the width direction can be accurately corrected and the sheet can be moved to the same position as that for forming the first color image.
Meanwhile, in recent years, a various sheet types such as coated paper, embossed paper, extremely thick paper and extremely thin paper are becoming popular to be used for image forming apparatuses. Accordingly, in addition to high productivity, skew-feeding correction at high speed and high accuracy has been desired for image forming apparatuses in order to be capable of supporting every type of sheets for using.
In order to achieve skew-feeding correction at high speed and high accuracy, a skew-feeding correction portion of an active skew-feeding correction type to correct skew feeding while conveying a sheet without stopping once has been proposed in Japanese Patent Application Laid-Open 2001-39546.
FIG. 11 is a view which illustrates the configuration of the skew-feeding correction portion of the active skew-feeding correction type in the related art. Pairs of skew-feeding correction rollers 21, 22 are illustrated in FIG. 11. The pairs of skew-feeding correction rollers 21, 22 respectively include a skew-feeding correction drive roller 21a, 22a shaped respectively to have a cutout portion at a part of the circumference thereof. A pair of registration rollers 30 are movable in the width direction and include a registration drive roller 30a shaped to have a cutout portion at part of the circumference thereof.
Activation sensors 27a, 27b to detect sheet skew feeding are arranged respectively at the upstream side in the sheet conveying direction of the pairs of skew-feeding correction rollers 21, 22. Skew-feeding detection sensors 28a, 28b are arranged respectively at the downstream side in the sheet conveying direction of the pairs of skew-feeding correction rollers 21, 22. Further, a registration sensor 131 to detect a sheet top end is arranged at the downstream side in the sheet conveying direction of the pair of the registration rollers 30. A lateral registration sensor 35 detects a sheet side end position in the width direction. Further, a registration HP sensor 32 and a registration shift HP sensor 34 are provided. Skew-feeding correction HP sensors 25, 26 respectively detect the home position (HP) of the pairs of skew-feeding correction rollers 21, 22.
With the skew-feeding correction portion as configured as described above, when the activation sensors 27a, 27b and the skew-feeding detection sensors 28a, 28b detect a sheet top end, driving of the skew-feeding correction motors 23, 24 is started in accordance with the detection timing. Accordingly, as illustrated in FIG. 12A, the pairs of skew-feeding correction rollers 21, 22 are rotated, so that skew-feeding correction of a sheet S is performed while conveying the sheet S. Next, as illustrated in FIG. 12B, the skew-feeding correction drive rollers 21a, 22a are positioned as the cutout portion facing to the sheet S. Then, the correction of the top end registration and side end registration is performed by the pair of registration rollers 30 in the state that nipping of the sheet S by the pairs of skew-feeding correction rollers 21, 22 is released.
That is, when the top end of the sheet S is detected by the registration sensor 131, a registration motor 31 is driven so as to align the image position on the photosensitive drum (not illustrated) and the top end position of the sheet S and controls rotation of the pair of registration rollers 30. Further, a lateral registration motor 33 is driven based on the detection signal from the lateral registration sensor 35 and the pair of registration rollers 30 are laterally moved so as to align the image position on the photosensitive drum and the position in the width direction of the sheet S. In this manner, the position of the sheet S can be accurately corrected against the image on the photosensitive drum and sheet conveying can be repeatedly performed thereafter.
With the skew-feeding correction portion configured as described above, position correction can be performed at high speed and high accuracy while supporting a variety of sheet types. As a result, the sheets are repeatedly conveyed at high speed to the same position at the downstream side of the skew-feeding correction portion. Therefore, especially when the sheets having unprecedented large burrs and high stiffness are continuously passing for a long time, there arises a problem that the surface layer of the photosensitive drum is shaved since the sheets are conveyed to the same position of the photosensitive drum located at the downstream side of the skew-feeding correction portion.
Similarly, after passing through the photosensitive drum, the sheets are conveyed to the same position of a fixing roller of a fixing portion to fix a toner image formed on the photosensitive drum onto the sheet. Here, the surface layer of the fixing roller is formed to be soft in order to support sheets such as embossed paper as well. Therefore, as illustrated in FIG. 13, there occurs a problem that a rubber portion constituting the surface layer portion of the fixing roller 118a is scratched to extremely shorten the life time when the sheets S are conveyed at the same position.
Accordingly, in the related art, sheets are conveyed in the condition that the sheet correction position in the width direction of the pair of registration rollers 30 is shifted by a predetermined amount while shifting the image to be formed on the photosensitive drum by the predetermined amount, for example. With this configuration, the surface layer of the roller is prevented from being scratched caused by the sheet conveying to the same position. In this case, it is preferable that the displacement amount of the sheet correction position is to be sufficiently large in order to sufficiently prolong the life time of the roller surface layer.
With an image forming apparatus in the related art having such a skew-feeding correction portion, in the case that the sheet processing portion is arranged as described above, sheets are discharged to the sheet processing portion as being largely displaced when the sheet correction position is largely displaced. Although the sheet processing portion is capable of performing sheet correction in the width direction to some extent, the possible range for correction by the sheet processing portion is limited.
Accordingly, when the sheet correction position is largely displaced, there may be a case that sheets cannot be moved to the position where alignment by the sheet processing portion is performed before arriving at the sheet processing portion. In this case, conveyance failure such as alignment failure and jamming is apt to occur, so that the sheet processing is affected. That is, when the sheet correction position is largely shifted in order to prolong the life time of the fixing roller, there arises a problem that the sheet processing by the sheet processing portion is affected.
To address this issue, the present invention provides an image forming apparatus which can prolong life time of a fixing roller without affecting sheet processing.